The compounds prepared by the process of the present invention are useful agents for the treatment of conditions such as allergic asthma because of their activity as prostaglandin antagonists. These compounds are disclosed to be present as components of a racemic mixture of compounds of the formula ##STR2## 7 or 8 fluorodibenzo[b,f]thiepin-3-carboxylic acid-5-oxide. Both the 7 and 8 fluoro derivatives have unusually high prostaglandin antagonist activity and as disclosed in copending application U.S. Ser. No. 210,082 filed Nov. 24, 1980 of Rokach, Rooney & Cragoe the racemic mixtures can be resolved into (+) and (-) optical isomers in which the bioactivity resides exclusively in the (-) isomer. In the prior application the method of resolution disclosed involves formation of diastereoisomeric amide using an optically active amine, followed by tedious separation of diastereoisomers using fractional crystallization, chromatography and HPLC. In this manner, racemic 7 or 8 fluorodibenzo[b,f]thiepin-3-carboxylic acid-5-oxide of formula I is separated into the biologically active isomers of formula II and the corresponding biologically inactive compounds of formula III pictured structurally hereinbelow ##STR3## Formula II includes 5(-)7-fluorodibenzo[b,f]thiepin-3-carboxylic acid-S-oxide and R(-)8-fluorodibenzo[b,f]thiepin-3-carboxylic acid-5-oxide. Formula III includes R(+)7-fluorodibenzo[b,f]thiepin-3-carboxylic acid-5-oxide and S(+)8-fluorodibenzo[b,f]-thiepin-3-carboxylic acid-5-oxide.